In the rotary drilling for oil and gas a drilling fluid is pumped down the drill pipe, out the drill bit, and up the annulus between the outside of the drill pipe and the drilled bore. The drilled fluid is examined at the well surface for gas, fluids and rock particles and are important in evaluating the possibilities of the subsurface formations for the production of oil and gas. However, it is critical to know the well depth from which the gas, fluids and rock were released by the drill bit. Therefore, a procedure has been employed to determine the lag time or delay interval between the time of the release of cuttings from the bottom of the well hole until they were pumped to the well surface for examination. By knowing the lag time, the annulus flow velocity and thus the depth of release could be calculated. The lag time is determined by pumping a "marker" (oats, carbide, etc. which could be identified by its appearance at the surface) down the interior of the drill pipe and back through the annulus to be observed, and identified at the surface. Time in minutes or pump strokes were used as a measure. This measurement included both dow pipe strokes or minutes (the passage of mud fluid down the interior of the drill pipe) and the "lag" (the passage of the mud fluid from the bottom to the surface in the annulus). Knowing the internal volume of the drill pipe, the down pipe strokes or minutes were calculated and then subtracted from the total circulation to obtain the lag from the bottom to the surface. The use of pump strokes meausrements was more accurate, since variations in pumping speed directly related to variations in the lag. Once a measurement in pump strokes was obtained the speed of the pump was irrelevant. However, a time base lag was inaccurate because it would not adjust to variable flow rates. Of course, as the depth of the bore hole increases both methods become inaccurate.
These methods above have been used with very few changes to the present. They are simple, easy to use, and the pump stroke counting switches and meters to count and keep totals are inexpensive. But the successful operation of the entire system was dependent upon the full-time presence of an observer-operator (mud logger). Two or three drilling mud pumps outputting different volumes per stroke pumped required the mud logger to calculate a lag for and switch counting instruments to each pump or combination of pumps used while drilling. In practice, this manual procedure has not been difficult but automatic switching of this sort would be very difficult, requiring considerable instrumentation.
Recently, industry efforts to develop an automatic, on-line, real time computerized mud logging system with the capability of gathering, storing and graphically presenting rate of penetration with mud gas accurately positioned to depth, all in an unattended situation, have had very limited success. The best efforts have been the pump stroke method which required elaborate expensive auto-switching instrumentation and the involvement of the driller or another rig crew member to keep the pump stroke counters operating and to route the proper counter(s) to the computer. Other efforts merely use a straight time lag which does not adjust for mud flow stoppages or variations. Elaborate auto-switching instrumentation on rig pumps introduces increased likelihood of equipment malfunction, and perhaps more important, the use of rig personnel in performing the switching is very unreliable.